In the area of semiconductors and photovoltaics, it is known to produce high purity silicon rods, e.g. according to the Siemens method in deposition reactors, which are also known as CVD-reactors. In this method, thin silicon rods are first placed in the reactors. Thereafter, silicon is deposited on these thin silicon rods during a deposition process in order to produce thicker rods. The silicon rods are first retained in clamping and contacting devices, wherein these clamping and contacting devices maintain the silicon rods in a desired orientation and provide for electrical contacting thereof. Two of the silicon rods are respectively connected at their free ends by electrically conductive bridges or bridges made from silicon material in order to form an electrical circuit via contacting devices located on the same side of the reactor. However, it is also possible to electrically contact the silicon rods their opposing ends, i.e. from above and from below.
The silicon rods are heated to a predetermined processing temperature at which deposition of silicon from a vapor or gas phase takes place on the silicon rods. The silicon rods are heated by resistance heating during the deposition process by a current flow at a predetermined voltage, where appropriate, the silicon rods may also be heated by an external heating unit. The deposition temperature usually lies between 900 and 1200° C. and particularly at about 1100° C. However, the deposition temperature may also be another temperature.
Initially, the silicon rods have a high resistance. In order to initiate an initial current flow in the silicon rod, at first a high voltage has to be applied. When doing this, the silicon rod changes from a non-conductive condition to a conductive condition, which is referred to as ignition in the following. The resistance markedly decreases after the ignition and particularly decreases when the temperature rises. For igniting the silicon rods and for subsequently heating the silicon rods up to the predetermined processing temperature, a sophisticated multi-stage power supply having different power supply units is required. Such a power supply is known e.g. from DE 10 2009 021 403 A or from DE 10 2010 020 740 A, which belongs to the same applicant as the present application but has not been published prior to filing the present application. In particular, a designated power supply unit for the ignition process is required.
The problem to be solved by the present invention is to provide a method and an apparatus decreasing the requirements in an ignition operation and an ignition unit in a CVD-reactor, respectively.